tzset

TZSET(3)                    Linux Programmer's Manual                   TZSET(3)



NAME
       tzset, tzname, timezone, daylight - initialize time conversion
       information

SYNOPSIS
       #include <time.h>

       void tzset (void);

       extern char *tzname[2];
       extern long timezone;
       extern int daylight;

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       tzset(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
       tzname: _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
       timezone: _SVID_SOURCE || _XOPEN_SOURCE
       daylight: _SVID_SOURCE || _XOPEN_SOURCE

DESCRIPTION
       The tzset() function initializes the tzname variable from the TZ
       environment variable.  This function is automatically called by the other
       time conversion functions that depend on the timezone.  In a System-V-
       like environment, it will also set the variables timezone (seconds West
       of UTC) and daylight (to 0 if this timezone does not have any daylight
       saving time rules, or to nonzero if there is a time during the year when
       daylight saving time applies).

       If the TZ variable does not appear in the environment, the tzname
       variable is initialized with the best approximation of local wall clock
       time, as specified by the tzfile(5)-format file localtime found in the
       system timezone directory (see below).  (One also often sees
       /etc/localtime used here, a symlink to the right file in the system
       timezone directory.)

       If the TZ variable does appear in the environment but its value is empty
       or its value cannot be interpreted using any of the formats specified
       below, Coordinated Universal Time (UTC) is used.

       The value of TZ can be one of three formats.  The first format is used
       when there is no daylight saving time in the local timezone:

              std offset

       The std string specifies the name of the timezone and must be three or
       more alphabetic characters.  The offset string immediately follows std
       and specifies the time value to be added to the local time to get
       Coordinated Universal Time (UTC).  The offset is positive if the local
       timezone is west of the Prime Meridian and negative if it is east.  The
       hour must be between 0 and 24, and the minutes and seconds 0 and 59.

       The second format is used when there is daylight saving time:

              std offset dst [offset],start[/time],end[/time]

       There are no spaces in the specification.  The initial std and offset
       specify the standard timezone, as described above.  The dst string and
       offset specify the name and offset for the corresponding daylight saving
       timezone.  If the offset is omitted, it default to one hour ahead of
       standard time.

       The start field specifies when daylight saving time goes into effect and
       the end field specifies when the change is made back to standard time.
       These fields may have the following formats:

       Jn     This specifies the Julian day with n between 1 and 365.  Leap days
              are not counted.  In this format, February 29 can't be
              represented; February 28 is day 59, and March 1 is always day 60.

       n      This specifies the zero-based Julian day with n between 0 and 365.
              February 29 is counted in leap years.

       Mm.w.d This specifies day d (0 <= d <= 6) of week w (1 <= w <= 5) of
              month m (1 <= m <= 12).  Week 1 is the first week in which day d
              occurs and week 5 is the last week in which day d occurs.  Day 0
              is a Sunday.

       The time fields specify when, in the local time currently in effect, the
       change to the other time occurs.  If omitted, the default is 02:00:00.

       Here is an example for New Zealand, where the standard time (NZST) is 12
       hours ahead of UTC, and daylight saving time (NZDT), 13 hours ahead of
       UTC, runs from the first Sunday in October to the third Sunday in March,
       and the changeovers happen at the default time of 02:00:00:

           TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"

       The third format specifies that the timezone information should be read
       from a file:

              :[filespec]

       If the file specification filespec is omitted, the timezone information
       is read from the file localtime in the system timezone directory, which
       nowadays usually is /usr/share/zoneinfo.  This file is in tzfile(5)
       format.  If filespec is given, it specifies another tzfile(5)-format file
       to read the timezone information from.  If filespec does not begin with a
       '/', the file specification is relative to the system timezone directory.

       Here's an example, once more for New Zealand:

           TZ=":Pacific/Auckland"

FILES
       The system timezone directory used depends on the (g)libc version.  Libc4
       and libc5 use /usr/lib/zoneinfo, and, since libc-5.4.6, when this doesn't
       work, will try /usr/share/zoneinfo.  Glibc2 will use the environment
       variable TZDIR, when that exists.  Its default depends on how it was
       installed, but normally is /usr/share/zoneinfo.

       This timezone directory contains the files
       localtime      local timezone file
       posixrules     rules for POSIX-style TZ's

       Often /etc/localtime is a symlink to the file localtime or to the correct
       timezone file in the system timezone directory.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────┬───────────────┬────────────────────┐
       │Interface Attribute     Value              │
       ├──────────┼───────────────┼────────────────────┤
       │tzset()   │ Thread safety │ MT-Safe env locale │
       └──────────┴───────────────┴────────────────────┘
CONFORMING TO
       SVr4, POSIX.1-2001, 4.3BSD.

NOTES
       Note that the variable daylight does not indicate that daylight saving
       time applies right now.  It used to give the number of some algorithm
       (see the variable tz_dsttime in gettimeofday(2)).  It has been obsolete
       for many years but is required by SUSv2.

       4.3BSD had a function char *timezone(zone, dst) that returned the name of
       the timezone corresponding to its first argument (minutes West of UTC).
       If the second argument was 0, the standard name was used, otherwise the
       daylight saving time version.

SEE ALSO
       date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can be
       found at http://www.kernel.org/doc/man-pages/.



                                   2012-03-25                           TZSET(3)